Fix the V4 valgrind integration

87a055bc8eee653a18d51f94a546cd452732223a broke the build and
introduced invalid writes to chunks outside of the existing
VALGRIND_DISABLE_ERROR_REPORTING block.

Fix the build by using MemoryManager::Data* instead of MemoryManager*
as the pool argument to VALGRIND MEMPOOL calls since the Data pointer
is protected.

Change-Id: I24136e25291570ade825dcd84a9568d8169a235b
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Support threading with QQuickRenderControl

Reorganize the rendercontrol example to demonstrate both the single
and multi threaded approaches.

A small helper function is introduced to the QQuickRenderControl API:
The QSGRenderContext has to live on the render thread. Previously there
was no way for applications to move it to the desired thread. This is
now possible.

Pass --threaded to the rendercontrol example to use a separate render thread.

[ChangeLog][QtQuick] QQuickRenderControl can now be used to render the Qt Quick
scene on a dedicated render thread, similarly to how the built-in threaded render
loop operates.

Task-number: QTBUG-42813
Change-Id: I01c3b2ffca8a174d9d2c267a51f2e484ed7b34b3
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>